The capacity of integrated circuits has increased primarily as the result of reductions in the size of features on a semiconductor chip. The lateral dimensions of features are generally defined by photolithographic techniques in which a detailed pattern is transferred to a reactive material by shining light through a photomask or reticle. In this disclosure, the term “photomask” is interchangeable with the term “reticle.” During the photolithography process, energy is applied to photoresist deposited on a wafer, where the energy application is controlled through the use of a patterned photomask. The exposure to the wafer is made by a step and repeat procedure. In this procedure, the wafer is moved and the steppers are used to move and repeat the pattern of the photomask over the wafer.
Printable defects on photomasks and reticles have historically been a source of defects that have reduced die yields. Printable defects in the photomasks are repeated many times over the surface of a semiconductor wafer since the photomask is stepped and repeated over the wafer. When defects are present on the photomask, this can result in substantial yield losses. Accordingly, it is important to detect as many defects as possible in the photomasks. Furthermore, defects can occur with repeated use of the photomask. Therefore, it is desirable to have an improved apparatus and method for inspection of photomasks.